There are four Kanjis.
Meaning:
5 | 6
7 | 8
Readings:
5: go, itsu, itsutsu
6: roku, mu, mui, muttsu/mutsu
7: shichi, nana, nano, nanatsu
8: hachi, ya, yô, yattsu/yatsu

Interesting fact: It depends on what you are counting, that you will use different readings and endings.